Aerodynamic vs. vehicle stabilty

[Stan Back]

Kid --

I think you misread him.  He's telling us that we ought to listen to those that know, not just guess, about these things.

Some of these problems don't show their ugly faces until upper speeds are reached.

Stan

[Cajun Kid]

Stan, you may be correct.

Charles

[racergeo]

  I met Eric (blue) at speedweek By recognizing his aircraft logo on his shirt. I will say that I believe he is the real deal. If a person wants to build a high speed car and is unsure if a certain design would be safe he would certainly be one to run your ideas by.


    You can sometimes always never tell what you least expect the most. George

[bvillercr]

We were waiting for him to stop by our pits.....never happened.

[jl222]

We were waiting for him to stop by our pits.....never happened.

  Emailed me he stopped by but we wern't there, said the car was well done but got busy helping 3 others in our class


                    JL222


      

[Blue]

.... I do agree with you about finding out "the point of no return"   but it would be costly to find that out (but less costly than an backwards car going airborne at 200 mph).  

I would think rather than spending research dollars to find that point, when in reality when a car gets to that point, the driver is not going to be able to react quick enough to employ any defensive counter measures to correct. ...

This is the problem, and it is clearly better to keep the car going straight than to spend a lot of time on blow-out panels and figuring out whether the car will lift between the time it gets sideways and the time it's going backwards.  Nascar spent tens of millions solving this problem for one car (COT).  In LSR we don't have that kind of budget.

Sorry for the people I didn't see on the weekend, I did walk the whole pits twice and introduced myself to anyone who wanted to take a stab at me if they were there.  I missed at least a dozen people I'd really like to spend some time with.  I met a lot of great people I'd missed in my last two trips, I look forward to meeting more next year.

This whole thread seems to have evolved into a discussion of steering control vs. yaw stability vs. how we keep cars from flying.  Several highly experienced sedan and roadster record holders have firmly stated that control trumps aero (at least up to the point of their current setups).  More than one have asked me if there is anything to make things more aero stable (meaning yaw-stable).  With a firm grasp on the rule book, here's a couple of suggestions:

1. Tuft the car.  OK, I'm flailing the deceased equine here.  I don't want anyone to take my word for anything, I want people to go down to the craft store, buy some rug yarn (3" long) and tape it to the spoiler, sides and roof and trunk of the car and then tell me what they see.  It's not pretty, and everyone who does will have a better understanding of how their B-to-C pillar and roof-to-trunk flow interacts with their "flat" spoilers and "flat" spill plates.  Relative to the flow off of the car, these things are anything but flat.

2. Test.  It only takes 20 to 40 mph for most of the separation and reversed flow effects to show up on the yarn.  It will be the same at 300.  Find some parking lot without law enforcement and spin some doughnuts.  Video and photograph the tufts at the yaw angle of the doughnut;  it's cheaper and faster than a wind tunnel with yaw capacity.  Put cheap tires on.  I would ask anyone who believes the flow going off the back of the car is fine to post pictures of their tuft tests.  I didn't see a single good aft closure during all of speed week.  If anyone disagrees, post some photos of the aft deck and bumper areas with tufts.  Even if you KNOW it's bad, do the test anyway: the data will help you make a better setup within the rules.  I have told the best test pilots in the industry where the reversed flow is and they believe me;  it doesn't matter.  They act on it when they see the yarn go backwards.

3. I saw lots of cars of all classes that allow wings with rules-limited "spill plates".  In the aero world, we call these things "end plates".  For a wing, their function is to make the wing more effective by minimizing tip vortexes.  For yaw, they should simply be as big, high, and as far aft as possible.  To this I would add that most car bodies would benefit from wider, not narrower, end plates.  If this creates too much spoiler downforce, de-camber the spoiler.  Keep the end plates at the maximum size and width that the rules allow.

4. Remember to add the yaw stability that comes from diffuser fences.  About half of the cars I saw used diffusers, some well, some not-so-well.  Verticals down there are tails too.  A few lakesters and roadsters were quite impressive in this area IM<HO.

Now we get into the tricky stuff.  3 teams have told me that they know they are unstable and, with varying levels of interest vs. derision, have asked me how to make it better.  Here goes:  Make the spill plates bigger, align them with local flow areas induced by the rest of the car (what works on a '53 Stude is different from what works on an '86 Camero), do everything you can under the car, and set the parachutes to eject up into clean air, not back into dirty air, so they can deploy before you are all the way around in a spin.  If anyone wants help, call me.  If anyone thinks I'm an idiot, fine, tuft the car during some doughnuts and post photos;  we all learn from tests.

The time to find out about stability is BEFORE the spin.

[Peter Jack]

Thanks for some really constructive information Eric.

Pete

[Cajun Kid]

Eric,  thank you very much, I actually understood what you said....

I think my side plates are to narrow (mounted to open with the trunk). I have a design from Hooley's 974 Stude and his are as far to the outside as can be, I may have to make entire new spoiler and side plates for Bonneville.

What I have works so far on pavement/concrete .

Here is a rear view (the chute deploys and acts well, so it must be in good air ??) I do have a video of that if you want to see and advise ?  The side plates could be relocated to the outside about 3" and mounted to the body.. or maybe I could leave this as is and just "add" a second set of side plates to the outside of these (making these more like jumbo diffusers??)



Charles

[Rex Schimmer]

Eric,
Very sorry that I missed you at Bonneville! the car I was working on proved to be more work than we expected. I also missed you at the Salt Talks and that was my fault, got talking to Linchy and Gary from Oz and lost concentration.

Thanks also for all of your aero words of wisdom in this thread and also our correspondence. Still planning to build a lakester with many of your thoughts incorporated into the design.

Rex

[jl222]

  Eric...the problem with good aft closer [ on a coupe or sedan ] is lift  

  On the spoilers SCTA allows due to the lower pressure under the spoiler and behind the car the air rushes up
 at high speed causing a turbulent wake. But the downforce is better than the streamlining.

  I tried to find out from those [ that should know] what kind of pressure differance there might be,but no luck

  On the 222 Camaro  there is 530 sq inchs of area if there was 1 lb of difference there would be 530 lbs of downforce.

               JL222

[bvillercr]

Here's a video of our wake.

http://www.youtube.com/watch?v=55XA0vrNNj0

[robfrey]

Gentlemen,
I spent enough time with Eric to know that he is the real deal. The cool thing about Eric is that he won't just let me take his word for anything. He makes me investigate it to come to my own conclusion. He has helped tremendously with our liner design.
I highly recommend his services!

[Blue]

OK, I set aside a little time and want to hit the four separate topics of the last 10 posts.

1. Spill plates.  I'm working for a Stude team right now, so I can't talk about what they have already paid me to come up with for performance.  What I saw long before speed week on a safety side is true for all cars using spill plates: Bigger is better, taller is better, and best of all is when they are aligned with the car sides so that they turn the entire side-of-quarter-panel-plus-spill-plate into one big vertical tail.  This does widen the spoiler and that leads to more separation drag, and this leads to...

2. Aft closure issues:  John, my comment was meant for streamliners and lakesters.  I know that sedans and roadsters are rules restricted in this area and I'm sorry if this sounded overly critical of sedans.  John is right that separation is inevitable in these classes, and that tapering off the upper deck to fill in the back would lift the car.  If someone is in a class that allows diffusers- DO IT.  Seth Hammond did a nice job of covering his chute tubes, it would be even better if the closure was integrated into the whole design like Bub-7.  If we're in sedans and roadsters, there are techniques that are within the rules.  I wish I had answered this before speed week because now I can't talk about some of this.  What I can say is that a lot of the spoilers I saw were "over-cambered" leading to more drag than needed.  You guys all have a good feel for the traction you've got, it would be good to get some weight-on-wheels data to do better tuning of the spoiler angle.  Then you can actually put marks on the end plates and make a concious decision every run on how much force you want vs. the extra drag.  This year, the salt got awfully soft between the 3.5 and 5 mile.  Under firmer conditions it would be practical to flatten out a little and gain some speed.  This means we need course reports, like seeing the size of the roostertail behind Speed Demon triple at the 4 mile marker.  That was some soft salt.  Back to the closure, that leads to:

3. Bvllercr's video: A little tough to do detailed aero from the dust, here goes.  First, there is an unstable shedding vortex off of the front tire.  This is responsible for the perfectly spaced dust clouds going down the side of the car.  This is an important thing to minimize because it disturbs the flow on the side of the quarter panel and since the left and right vortexes will seldom be synchronized, it induces a yaw instability.  Next, the aft separation is pulling the dust from the wake forward into the lower spoiler/ bumper area.  The vortexes from the rear tires are messing with this too.  Look at some of the new high mpg hybrids for ideas on how to organize the rear tire vortex; it's one of the tricks to getting 60 mpg and it works for LSR too without creating lift.  The air in the separated area is literally going forward, not back, and this leads to...

4. Chute position:  I see a lot of chutes that eject straight back into dirty air and this is not good.  The chute is ejected into air that is recirculating and this delays the chute opening.  In some cases, the bag drags on the ground before it opens.  A lot of teams have theirs mounted at an up angle, and this is better, but the angle often is below the angle of the spoiler, so the chute is still buried in dirty air.  Either arrangement will work fine when we're straight, let's worry more about how it works when "oh-shoot-we're-spinning-I-pulled-the-handle-and-why-hasn't-the-darned-thing-opened-yet???"  A Honda red hat holder narrated his spin video for me and said that chute opening seems fast during a clean run and awfully slow when spinning.    Mount it vertically.  Yes, I said shoot the chute straight up.  As soon as the drogue clears the aft lip it's in clean air and it will deploy in a fraction of the time that it takes in dirty air.  Use a d-bag too, if you're spinning a d-bag will help prevent the chute from fouling on the car.  Bob Stroud and I talked about this at length and the more vertical the better.

One funny note from speed week, I was the guest of a mod roadster team (one of the more challenging classes to do aero work in) and was asked how effective the long nose is.  I replied that the car would be much faster running backwards: i.e. round in front and pointy behind.  I said this might be a fun thing to do and one of the legends of the salt, Marlo Treit, replied: "You're about 50 years too late."  Apparently someone did this way back in the day at Elmo and after two years of whipping everyone else the SCTA made a rule that the bodywork must be run in the stock direction.

[hotrod]

You don't even need to drive the car to get basic data if you live in an area that has steady straight line winds and can find an open parking lot or scenic overlook.

These tuft tests were done on a scenic over look west of Denver during a chinook wind event with steady winds of 70-80 mph with gusts to 110.

(Yes it was a tough day to be outside -- the storm blew down 150' of board fence at my house while I was doing this breaking the fence posts off at the ground).

At the  time I was trying to achieve a local drag strip record for the WRX with the stock turbo and OEM 2.0 liter engine (I eventually got it with my nearest competitor in this unofficial competition being 2 mph slower in his trap speed).

These photos were to determine best air dam width and to understand the cars aerodynamics.

Photo #1 was to figure out frontal area and see what the "air sees"

Photo#2
Then I went out to the scenic over look when the weather cooperated and tufted the car.

Photo #3
Side view, note the strong vortex that forms on the side of the passenger compartment where local flow is nearly vertical on the drivers side window due to the sheet of air that dumps off the side of the windshield then rolls up into a tube behind the side view mirror.

Photo #4
After thinking about the results of this test I went out and did a more complete tuft test in low winds (about 20-30 mph) to better understand the flow on the side and rear of the car.

[hotrod]

Then I borrowed a page from the glider folks and did some oil tests. To do these you put spots of used engine oil on the car body, and drive it at about 60-70 mph for a mile or so, then pull over and quickly take a picture of what the oil does as the air flow in the boundary layer blows the oil across the body surface.